In an ultrasonic diagnosis apparatus functioning as a subject information obtaining apparatus, a spatial resolution in a depth direction in a case where image data is formed through a pulse echo technique can be generally represented by (nλ)/2 when a wavelength of an ultrasonic wave is set as λ and a transmission wave number is set as n. For example, the spatial resolution in the depth direction is approximately 0.13 mm in a case where an ultrasonic wave having a center frequency at 12 MHz is transmitted for two wavelengths.
A description will be provided of the pulse echo technique. First, when an ultrasonic pulse (elastic wave) is transmitted to a subject, an ultrasonic wave is reflected and returned in accordance with an acoustic impedance difference within the subject. Next, this reflection wave is received, and image data is generated by using a reception signal of the reflection wave. Typically, an envelope of the reception signal is obtained, and this envelope is converted into a luminance value to generate the image data. By repeating the transmission and reception of the ultrasonic wave in plural directions or positions within the subject, it is possible to obtain luminance information on plural scanning lines in the directions in which the ultrasonic wave is transmitted and received. Imaging within the subject is enabled by disposing the luminance information on the plural scanning lines.
It is noted that in the ultrasonic diagnosis apparatus, plural conversion elements configured to convert the ultrasonic wave into an electric signal are generally used, and a temporal shift is added to reception signal waveforms between the respective elements to be focused within the subject in both the transmission and the reception.
As described above, the spatial resolution on the order of approximately 0.13 mm in the depth direction can be realized by using the pulse echo technique, but a higher spatial resolution is demanded. For example, if a layered structure of a blood vessel wall of the carotid artery can be observed in further detail, the observation may contribute to an early detection of hardened arteries and the like.
As a technology for improving the above-described spatial resolution in the depth direction, NPL 1 illustrates a result obtained by imaging a layered structure of a blood vessel wall by applying a frequency domain interferometry (FDI) and a Capon technique corresponding to an adaptive signal processing. By applying the FDI and the Capon technique to the reception waveform, it is possible to further improve the spatial resolution in the depth direction (scanning line direction). It is however noted that within a range of the signals in the depth direction cut off for carrying out the FDI processing (within a processing range), plural reflection layers are supposed to exist. Also, plural reflection layers from adjacent reflection layers are likely to mutually have a high correlativity. If the adaptive signal processing such as the Capon technique is directly applied to the reception signals of the plural reflection waves having the above-described high correlativity, an unexpected operation such as a cancellation of a wanted signal occurs. To reduce (suppress) the influence from the signals having the above-described correlativity (correlative interference wave), by using a frequency averaging technique in combination, it is possible to apply the FDI and the Capon technique to the reception signals of the reflection waves.